The present invention relates generally to power systems, and more particularly to limiting AC and DC currents in power systems by using Hall effect devices.
Means for limiting high AC and DC currents are well known in the art. Limiting high DC currents sufficiently rapidly is difficult, since the circuit DC current could double or triple in a few msec. The current limiting apparatus must therefore respond quickly at low or moderate circuit overload. In a typical case, for example, a 700 volt DC supply will operate with a current output normally in the range of 50 kiloamperes (kA). A short circuit may cause the current to increase at a rate of 25 kA/msec up to as high as 200 kA. Unless this current is quickly limited, circuit damage from burning caused by excess heat generation is likely. Magnetic forces between adjacent conductors carrying such currents in opposite directions also increase as the square of the current, thereby providing a second probable cause for circuit damage. Existing methods for limiting high AC and DC currents include the use of fuses, circuit breakers and crowbar protective circuits.
The difficulty with most fuses is that they require a high overload current, on the order of 30 to 50 times the normal circuit current, to blow within one or two milliseconds. This is because the fuses are designed to carry large continuous currents under normal operation, being a series element in the circuit between the power source and the load. Such fuses also are disadvantageous, since they have poor voltage clearing capabilities. Voltage clearing is the process of clearing away conductive matter in the fuse so that arcing is not enabled through the remnant material once the fuse has blown. Fuses that are required to conduct high currents must have a very low resistance to minimize heating and power loss during normal operation. This requires that the fusible element have a large cross section, which does not clear well.
Circuit breakers are also disadvantageous, since they are mechanical devices, tend to be too slow and may not be capable of breaking circuits having high currents in the range of many kA due to arcing across the breaker contacts, or other problems. Crowbar protective circuits are similarly undesirable, since they add to the overload on the power supply and the power bus during a fault condition. In general, considerations of mountings, lead breakouts, connections, resistance, and space requirements also make such devices poor choices as high power current limiting devices.
In addition, magnetic forces resulting from the high currents in such circuits may affect circuit breaker or crowbar protective current switches adversely. Many switch designs in the prior art do not take into account strong magnetic forces, and thus risk malfunction of the switch.
Arc clearing is a problem with most of these switches, such that auxiliary commutating circuits to quickly generate current zeros are usually required. This causes the switches that interrupt a circuit suddenly to generate high transient voltages which inhibit the suppression of arcing during switch contact opening. These high voltage transients may also cause damage to other parts of the system, e.g., to the power supply insulation.
Other prior art limiting devices are mechanical in nature such that physical movement and control of contacts and/or sensors are required. This is not desirable because such devices are expensive and unreliable.
Therefore, there remains a need for systems and methods for limiting current that avoids the above described drawbacks of the prior art.
The present invention is directed to a current limiting apparatus comprising a first contact, a second contact, and a Hall effect device disposed in electrical contact with the first contact and the second contact. The first and second contacts are each capable of generating an axial magnetic field for the Hall effect device.
According to one aspect of the invention, the first and second contacts are substantially identical.
According to further aspects of the present invention, each of the first contact and the second contact has a base terminal and at least one connection surface, and the base terminal or the connection surface(s) is (are) in electrical contact with the Hall effect device.
Another embodiment of the present invention is directed to a method of limiting current in a power system comprising providing current to a Hall effect device, generating an axial magnetic field, and subjecting the Hall effect device to the axial magnetic field to limit the current.
According to further aspects of the present invention, generating the axial magnetic field comprises providing current to a first contact and a second contact, and the Hall effect device is disposed between and in electrical contact with the first contact and the second contact.
According to further aspects of the present invention, subjecting the Hall effect device to the axial magnetic field to limit the current comprises changing the impedance of the Hall effect device using the axial magnetic field.
The foregoing and other aspects of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.